This invention is founded in improvements in decalcomania, customarily called decals and, in particular, to stretchable heat release decalcomania which can be applied to surfaces of complex contours.
Decals used extensively in commerce for decorating glass and ceramic articles can be generally categorized into three groups or types depending upon their construction and their mode of application; viz., water slide-off, heat release, and cold or pressure release decals. Those decals have commonly been employed not only for applying designs and decorations to surfaces of articles, but also for applying continuous coatings that can serve either a decorative or a functional purpose.
The construction of the commercially available decals has limited their application to articles of relatively simple geometric shapes. That limitation becomes particularly restrictive where it is desired to apply a continuous, unbroken coating over a relatively broad surface area. Hence, it is extremely difficult to avoid developing wrinkles, air entrapment, distortions, and other physical defects which result from efforts to uniformly conform the coating to the surface of an article.
In a number of decals the underlying source of this shape limitation resides in the backing layer of carrier for the decal. To illustrate, where paper comprises the backing layer and it is necessary for that layer to remain in contact with the design layers during application of the decals, then it is apparent that this backing layer will severely restrict the ability of the decal to conform to article surfaces of complex geometries. Conventional heat release decals provide examples of that situation. Hence, their application is normally effected by lightly pressing the decal against a heated substrate, the heat therefrom activating an adhesive top coat to thereby cause the decal to adhere to an article surface, while concurrently melting a wax-based release layer to effect release of the backing layer. Pressure release decals are applied in a similar manner, but no heat is required because the top coat is a pressure sensitive adhesive and release of the backing layer is occasioned through the use of a silicone release coating on the surface of the backing layer. The use of a silicone release coating assures that the adhesion of the decal to the article surface will be greater than the adherence to the backing layer, thereby guaranteeing that complete transfer of the decal to the article surface can be accomplished.
One technique which has been devised to overcome the surface shape limitations encountered with conventional decals has involved a two-step process: first, transferring the design layer to a lower durometer silicone transfer pad; and then, second, transferring the design to the surface of an article by pressing the transfer pad with the design thereon against the article surface. That technique can be effected successfully if the materials of construction of the design layer are carefully selected to demonstrate not only the proper characteristics to hold the design together during transfer, but also sufficient flexibility to conform to the shape of the article surface, and a balanced adhesion between the pad and the article surface. That technique is not applicable, however, where a coating to perform an operational function is desired because it conventionally results in a wax release coating being under the decal after application thereof, that coating imparting extremely poor durability to the decal unless fired at high temperature to remove the wax, such as is done with ceramic and/or glass-containing decals where the ultimate design layer is to be a sintered or fused pigmented glass flux.
Another technique devised to overcome the surface geometry limitations experienced in the use of conventional decals utilizes a heat release decal of the type described in U.S. Pat. No. 4,477,510 (Johnson et al.) wherein the backing or carrier layer employed is a stretchable film, rather than relatively rigid paper. In that technique the film is stretched to conform to complex surface geometries and the decoration released under light pressure when brought into contact with the surface of an article and heat is applied to melt a wax release layer, thereby avoiding the need for high pressure during application.
As defined in that patent, the decals consisted of a three ply laminate: (a) a uniformly stretchable carrier or support; (b) a release layer deposited onto that carrier; and (c) a design layer or decoration deposited onto the release layer. The carrier and the decoration carried thereon can be stretched or otherwise shaped to conform the decoration to the geometry of the article. When the decal is brought into contact with the article and heat is applied, the decoration releases (separates) from the carrier and adheres to the article. The carrier is thereafter disposed of.
As defined more specifically, the decals of Johnson et al. consisted of a carrier or support formed form a disposable stretchable film of low density polyethylene, a release layer deposited onto the carrier formed from an organic wax, and a design layer deposited on the release layer as a cohesive film formed of a heat-processable thermoplastic ink having a melting point higher than that of the release layer. Each of the carrier, the release layer, and the design layer was prepared from materials which did not migrate into each other during formation of the decal and upon application of the decal to the article, and each of the release layer and the design layer was stretchable with the carrier.
As can be observed, this technique offers the distinct advantage in that the release wax is on the top surface of the decal after transfer such that, consequently, it does not interfere with the decal's durability for those applications wherein the decal will not be subsequently fired.
Whereas, in theory, there are pressure release decal equivalents to the above-described techniques for heat release decals, the pressure release decal approach has been found to be more difficult to effect successfully because of the requirement to formulate the design layer with pressure sensitive materials and to control precisely the properties of the silicone release material. This latter situation is especially difficult in the above-described technique utilizing an intermediate transfer pad inasmuch as the pad must exhibit an affinity for the decorating layer intermediate between that for the backing layer and that for the article surface. This situation is further complicated by the fact that the surface energy of the transferring pad, which energy dictates the adherence of the decorating layer, does not remain constant during continuous repeated process operation.
By being slid off the carrier layer (after soaking in water to dissolve the layer between the design layer and carrier) and then being conformed to the surface of the article by manually smoothing out the decal onto the surface of the article, water slide-off decals circumvent the problems inherently imposed by the backing or carrier layer in the heat sensitive and pressure sensitive decals. Nevertheless, water slide-off decals demand considerable skill when applying to articles of complex shapes, but work reasonably well in forming irregular patterns on articles of relatively simple geometric shapes. The ability to produce continuous coatings, however, is quite shape limited, inasmuch as it is extremely difficult to avoid the development of wrinkles, creases, distortion, etc. Furthermore, the lacquers customarily employed in the construction of water slide-off decals to maintain the design layers intact during application to the surface of an article comprise materials such as nitrocellulose, acrylics, cellulosics, etc., which demonstrate limited extensibility and, thereby, also further restrict the ability of the decal to conform to complex surface geometries.
In summary, all of the above-described decal constructions and application techniques suffer one or more shortcomings in the capability of transferring continuous functional coatings to articles of complex shape, i.e., coatings wherein the decals will not be subsequently fired. Those deficiencies become more obvious and even more restrictive where maintaining an optical surface quality on the article is required. Hence, to maintain optical quality, the transferred coating must be essentially defect-free, homogeneous, of uniform thickness, and have a smooth surface. To assure the latter characteristic, it has been found that only extremely smooth polymer films make satisfactory backing or carrier layers; paper, including coated paper, have been found to be unsatisfactory. Thus, the materials of decal construction and the application technique must be so devised that completely uniform transfer of the decal is attained with no optically unsatisfactory defects being introduced which could result from non-uniform film thickness, entrapped air, and the like.